FIG. 51 shows a radiation image captured by a radiographic imaging apparatus.
In order to observe a lung field of a subject reflected in such a radiation image, it is necessary to adjust the contrast of the lung field to improve the visibility of the lung field.
In the radiation image, various portions of the subject, such as bony parts, other than a lung field, are reflected. The bony part of the subject is darkly reflected in the radiation image because the body part hardly transmits radiation. Furthermore, the portion outside the contour of the subject reflected in the radiation image is a portion where the subject is not reflected but the air is reflected. The portion where the subject is not reflected is brightly reflected in the radiation image because there is nothing to absorb radiation. In the radiation image, the lung field is brighter than the bony part of the object but darker than the portion outside the contour of the subject, which is the portion where the air is reflected.
The lung field of the radiation image totally looks to be filled with a gray color with poor contrast. This is because the pixels located in the lung field reflected in the radiation image have similar pixel values.
When the contrast adjustment is executed for the entire radiation image for the purpose of increasing the visibility of the lung field, the contrast adjustment is also executed for the portions of the radiation image in which the bony parts and the air are reflected. Although such a contrast adjustment may increase the visibility of the radiation image as a whole, as far as the lung field is concerned, there is not so much improvement in visibility. The lung field after the contrast adjustment is still in a totally poor contrast state. This is because that, in the case of expressing the contrast of the lung field, low pixel values are used to express the bony parts of the subject and high pixel values are used to express the portion where the air is reflected, and therefore the lung field should be expressed with the remaining moderate pixel values.
Therefore, a method of making a contrast adjustment only on a lung field in a radiation image has been conventionally conceived. According to this method, since the lung field can be expressed with more various color tones, the visibility of the lung field is assuredly increased. In this method, trimming is performed to extract the lung field in the radiation image, and a contrast adjustment is executed on the trimmed image in which the lung field is largely reflected. In the trimmed image, dark bony portions of the subject and bright portions where the air is reflected are excluded, so the trimmed image is not affected by these portions.
A conventional lung field trimming method will be described. In a conventional method, first, edge enhancement processing is performed on a radiation image as shown in FIG. 52. The edge enhancement processing is image processing that can be realized by spatial processing such as differentiation processing and darkly expresses portions where the pixel value in the image changes extremely. By using this edge enhancement processing, the contour of the subject can be grasped. The edge enhancement is detailed in Patent Document 1.
According to a conventional method, trimming processing of trimming the area including a lung field as shown in FIG. 53 is performed based on a contour of a lung field. In the trimming processing at this time, the lung field area is recognized based on the edge enhanced image, and the lung field including its periphery is roughly extracted from the radiation image.
A conventional trimming processing using the edge enhanced image is performed by generating the profile as shown in FIG. 54 based on the edge enhanced image. The profile is a profile showing the relation between the total value obtained by adding pixel values of each pixel array obtained by decomposing the edge enhanced image into pixel columns each having a width corresponding to one pixel and the position of each pixel array. The profile is generated by decomposing the edge enhanced image into pixel columns extending in the vertical direction, and shows the relation between the position in the image in the horizontal direction and the total value of the pixel value.
Observing this profile, it is found that there are parts where the total value is extremely high. This part shows an existence position of a pixel column where there exist many pixels having high luminance pixel value in the edge enhanced image. In the edge enhanced image, the high luminance pixel value means the portion where the change of the pixel value is severe in the original image. Therefore, by finding the part whose total value is extremely high in the profile, it is possible to know the portion in the original image where the pixel is suddenly changed.
In reality, in the original image, the pixel value in the vicinity of the lung field contour is a part where the pixel value changes dramatically. The part where the total value of the profile is extremely high represents the position of the lung field contour in the original image. The portion where the total value appeared in the profile is extremely high represents the right end or the left end of the lung field contour. By analyzing such a profile, a trimmed image roughly cut out the lung field area from the original image can be obtained.
The position of the upper end of the lung field contour and that of the lower end thereof can also be obtained by analyzing the edge enhanced image.
By performing the contrast adjustment by using the luminance information of the rectangular area of the lung field searched as shown in FIG. 55, the genuineness of the lung field is assuredly improved.